ABSTRACT Mutations in the merlin (NF2) tumor suppressor gene cause the benign tumor disorder, Neurofibromatosis type 2 (NF2). This disorder predisposes individuals to develop bilateral vestibular schwannomas (VS) that cause progressive hearing loss and can cause life-threatening brainstem compression. Because surgical removal of a VS often causes deafness, facial paralysis, and imbalance, there is a need to develop drug therapies to slow or prevent VS growth and preserve nerve function. We have worked to establish an in vitro and in vivo drug screening platform to identify novel compounds as well as FDA-approved drugs that can be developed/repurposed for VS therapies. Toward this goal, we have created mouse and human merlin-deficient Schwann cell lines and optimized their use in 384-well high-throughput and high-content assays in order to screen large compound/drug libraries using robotic platforms. This approach identified several phosphoinositide- 3 kinase (PI3K) inhibitors that selectively reduce viability of mouse merlin-deficient compared to wild-type Schwann cells with nanomolar IG50. This initial finding was confirmed in human merlin-deficient Schwann cell lines for multiple PI3K, dual PI3K/mTOR and PI3K/HDAC inhibitors. Because PI3K plays a critical role in cell proliferation, survival, and invasion, there are currently 15 different PI3K inhibitors in clinical trials for various blood cancers and solid tumors. The first in class PI3K inhibitor (idelalisib) was approved in 2014 for leukemia. In this proposal, we advance our findings by conducting a systematic screen of PI3K pathway inhibitors in vitro and in vivo. The aims of this proposal are to: 1) profile a library of PI3K pathway inhibitors for efficacy in reducing viability of human merlin-deficient Schwann cell lines and primary human VS cells; 2) test efficacy of the advanced PI3K inhibitors to slow graft expansion and preserve hearing and balance in a novel rat xenograft model, and 3) conduct phenotypic, kinome, and transcriptome analysis to reveal the molecular signatures and adaptive changes of the cells and grafts to the inhibitors. We expect to obtain the necessary pre-clinical data to support the potential use of PI3K inhibitors in patients with NF2-associated VS.